We'll be in this country for another 2 years, and it's not like she'll be returning to the Kingdom of Asura the moment she graduated. She'll probably be here for another 5 years. That's why, umm..."
Sylphy doesn't seem to be planning on quitting her job as a guard. From the fact that she won't simply quit, you can feel the strength of her bonds with Ariel-sama and Luke.